7970, since MSI neutered the Lightning cards because of nVidia's ask, you can't overvolt the Lightning any more officially.
Yes, there's a built in hack for it, but Unwinder said he won't be developing the functionality any more and it's still a bit buggy.

Yup this Matrix card sold like hotcakes upon release and both NCIX and Newegg are out of stock without word of when they might be stocked again. Looks like AMD may be a victim of their own success. The only other option now is nVidia.

Edit: Used shopbot and found an etailer that claims to have 5 in stock.